Polyisocyanurate adducts of polyisocyanates are well-known intermediates used in the preparation of high performance urethane coatings, paints, and films. These adducts provide improved physical properties when used in such applications, as compared to difunctional isocyanates such as toluene diisocyanate. In addition, these adducts provide reduced volatility and an associated reduced toxicity hazard during use, as compared to toluene diisocyanate.
Processes for preparing these adducts are well known. Examples illustrative of these processes can be found in U.S. Pat. Nos.: 4,220,728; 4,265,798; 4,324,879; and 4,412,073. Generally, the prior art processes involve adding a catalyst which promotes the isocyanate to isocyanurate (also known as "trimerization") reaction to the precursor isocyanate, optionally in the presence, but usually in the absence, of a solvent, allowing the reaction to proceed to the desired extent and then stopping the reaction with a suitable quenching agent which destroys the activity of the catalyst.
After the residual, unreacted precursor isocyanate is removed, the resulting material, in the case where the precursor isocyanate is a diisocyanate, is a mixture of oligomers composed of 3, 5, 7, etc. precursor diisocyanate molecules joined by 1, 2, 3, etc. isocyanurate rings. Usually, this mixture is simply called "trimer".
In the case where the precursor isocyanate is polyisocyanate, the reaction is generally stopped well before all the isocyanate groups have been converted to isocyanurate groups because, otherwise, the resulting product would be an unusable polymer having a very high (theoretically infinite) molecular weight and viscosity. However, the cost of equipment and energy to remove residual, unreacted precursor isocyanate dictate that the reaction not be stopped too soon. Generally, the reaction is run to more than 10% conversion but less than 50% conversion. The preferred range is between 20 and 35%. The reaction is typically stopped using a quenching agent. The reaction between conventional trimerization catalysts and quenching agents typically results in the formation of an insoluble product which is typically removed by filtration using a filter aid.
Unfortunately, both the quenching agent and the filter aid increase the likelihood of introducing undesirable impurities into the product. Accordingly, new processes for producing trimers that do not employ a quenching agent and filter aid(s), and employ fewer process steps than prior art processes, would be highly desired by the trimer manufacturing community. Heretofore, such processes have not been known to the knowledge of the present inventors.